Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization on n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, polymetallic catalysts have been commercialized by the addition of other metallic components to platinum as promoters to further improve the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, tin, and the like. Platinum-rhenium catalysts, by way of example, possess admirable selectivity as contrasted with platinum catalysts, selectivity being defined as the ability of the catalyst to produce high yields of C.sub.5.sup.+ liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., methane and other gaseous hydrocarbons, and coke.
Reforming reactions are both endothermic and exothermic, the former predominating, particularly in the early stages of reforming with the latter predominating in the latter stages of reforming. In view thereof, it has become the practice to employ a plurality of adiabatic fixed-bed reactors in series with provision for interstage heating of the feed to each of the several reactors. There are two major types of reforming. In semi-regenerative reforming, the entire unit is operated by gradually and progressively increasing the temperature to compensate for deactvation of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In cyclic reforming, the reactors are individually isolated, or in effect swung out of line by various arrangements, the catalyst is regenerated within the regenerator circuit to remove the coke deposits, and then reactivated while the other reactors of the series remain on-stream. A "swing reactor" temporarily replaces a reactor which is removed from the series of in circuit reactors for regeneration and reactivation of the catalyst, and is then put back in series. Such an operation is described by reference to U.S. Pat. No. 4,166,024 which issued Aug. 28, 1979, to George A. Swan, particular reference being made to the figure and the description thereof at Column 5, beginning at line 61 and continuing through Column 7 at line 21; which disclosure is herewith incorporated as part and parcel of the present application. In either type of reforming, hydrogen is produced in net yield, the product being separated into a C.sub.5.sup.+ liquid product, e.g., a C.sub.5.sup.+ /430.degree. F. product, and a hydrogen rich gas a portion of which is recycled, generally after passage through a recycle gas drier to the several reactors of the process unit. In semi-cyclic reforming the reforming unit contains both reactors which remain in operation throughout the length of the operating cycle, i.e., from start-up to shutdown for regeneration, and reactivation of the catalyst, and reactors which are replaced for regeneration and reactivation of the catalyst by swing reactors throughout the length of the operating cycle.
Essentially all petroleum naphtha feeds contain sulfur, a well known catalyst poison which can gradually accumulate upon and poison the catalyst. Most of the sulfur, because of this adverse effect, is generally removed from feed naphthas, particularly by hydrofining, or hydrogen treating. In use of the more recently developed polymetallic platinum catalysts wherein an additional metal, or metals, hydrogenation-dehydrogenation component is added as a promoter to the platinum, it has become essential to reduce the feed sulfur to only a few parts, per million parts by volume of feed (ppm). For example, in the use of platinum-rhenium catalysts it is essential to reduce the sulfur concentration of the feed well below about 10 ppm, and preferably well below about 2 ppm, to avoid excessive loss of catalyst activity, and C.sub.5.sup.+ liquid yield. It is thus known, e.g., that as little as 2 ppm of feed sulfur over the length of the operating cycle can lower catalyst activity by as much as 15%, and C.sub.5.sup.+ liquid yield by as much as 1 LV %. For this reason, generally, the product from the hydrofiner is stripped, and then fed to a guard chamber, e.g., one which contains nickel oxide, cobalt oxide, zinc oxide or the like, to essentially completely remove any residual sulfur from the feed.
The role of sulfur on the catalyst presents somewhat of an anomaly because the presence of sulfur in the feed can adversely affect the activity of the catalyst and reduce liquid yield; and yet, sulfiding of the polymetallic catalyst species, which is a part of the catalyst reactivation procedure, has been found essential to suppress excessive hydrogenolysis which is particularly mainfest, when a reactor is first put on-stream after regeneration and reactivation of the catalyst. Excessive hydrogenolysis caused by use of these highly active catalysts can not only produce acute losses in C.sub.5.sup.+ liquid yield through increased gas production, but the severe exotherms which accompany operation in a hydrogenolysis mode can seriously damage the catalyst, reactor, and auxiliary equipment. In semi-regenerative reforming, for example, it has been found that when the reactors of a unit which contain fresh, or regenerated, reactivated highly active rhenium promoted platinum catalysts are put back on-stream, the start-up period is characterized by an initial loss of catalyst activity and loss of C.sub.5.sup.+ liquid yield. The same phenomenon is observed in cyclic reforming. When a platinum-rhenium catalyst loaded reactor is reinserted in the multiple reactor series of the unit, albeit it contains a fresh catalyst, or a regenerated, reactivated, sulfided catalyst, there occurs an initial upset period when the catalyst activity and C.sub.5.sup.+ liquid yield of the unit is reduced. It has been observed that his effect is first noted in the reactor immediately downstream of the swing reactor which when first put on-stream contains a freshly sulfided catalyst. A quantity of sulfur is released when the freshly sulfided catalyst is contacted with the feed, the sulfur wave travelling downstream from one reactor to the next of the series. Concurrently with the sulfur wave there results a loss in C.sub..sup.+ liquid yield which, like a wave, also progresses in seratim from one reactor of the series to the next until finally the C.sub.5.sup.+ liquid yield loss is observed throughout the unit. Over a sufficiently long period after the initial decline in C.sub.5.sup.+ liquid yield loss, the C.sub.5.sup.+ liquid yield in the several reactors of the unit, and consequently the overall performance of the unit, gradually improves, though often the improvement is not sufficient to return each of the reactors of the unit, or unit as a whole, to its original higher performance level.
In the use of polymetallic catalysts it is essential to utilize good sulfur strategy, especially as contrasted with the use of the older, less sulfur-sensitive and conventional platinum catalysts. (1) One approach to the problem has been to maintain sulfur on the fresh, or regenerated, reactivated catalyst in the reactor as it is first swing on oil. In this approach, the catalyst is presulfided while in the regeneration circuit as by treatment to breakthrough with a sulfur-containing gas, e.g., hydrogen sulfide-in-hydrogen. The reactor, after the catalyst ispresulfided, is then swung on oil. (2) A second approach involves a consideration of the amount of sulfur in the feed and gas of the reactor circuit. In this approach the fresh, or regenerated, reactivated catalyst is not presulfided in the regenerator circuit but rather it is sulfided in situ within the reactor circuit by contact with relatively high hydrogen-sulfide containing gases. The high hydrogen sulfide in the reactor circuit, e.g., from about 0.6 to 10 ppm, sulfides the catalyst suppressing exotherms and rapid coking. (3) A third approach relates to maintaining an average, low sulfur level throughout the operating cycle. Pulses of sulfur are provided for the most part by sulfur releases from the regenerated, reactivated catalyst as the reactors containing the fresh, or regenerated, reactivated catalysts are swung on oil. These approaches have been moderately successful but further improvements are nonetheless needed.
It is, accordingly, the primary object of this invention to provide a new and improved process that will obviate these and other disadvantages of present catalyst sulfiding procedures for cyclic reforming units, particularly those employing highly active polymetallics, or promoted noble metal containing catalysts.
A specific object is to provide a novel operating procedure for cvyclic reforming units, notably one which will enhance catalyst activity and more effectively suppress C.sub.5.sup.+ liquid yield decline which is particularly acute when sulfur is present during reforming operations with metal promoted platinum catalysts, particularly rhenium promoted platinum catalysts.
These objects and others are achieved in accordance with the present invention embodying an improved process wherein, in the operation of a cyclic reforming unit, a sulfur-containing naphtha feed is first hydrofined to reduce the sulfur content of the feed, the product therefrom is then passed through a guard bed or reactor and contacted with a sulfur adsorbent to essentially completely remove the feed sulfur, and thereby maximize catalyst activity, catalyst stability and C.sub.5.sup.+ liquid yield, the improvement wherein at the time of, or just prior to, swinging on-stream a reactor which contains an unsulfided fresh, or unsulfided regenerated, reactivated sulfur-sensitive polymetallic platinum catalyst, the series of reactors is by-passed around the hydrofiner and guard chamber and product from the hydrofiner fed directly into the series of reactors to sulfide the catalyst.
In accordance with this invention, the amount of sulfur introduced into the reforming unit is optomized consistant with the dual objective of sulfiding the catalyst, and minimizing the average sulfur present in the unit during an operating cycle. During normal operation, i.e., between reactor swings, the reformer feed is desulfurized in the hydrofiner and guard chamber to essentially completely remove the sulfur. Hence, between reactor swings the reformer feed contains from about zero to about 2 ppm sulfur, preferably from about 0.01 to about 0.5 ppm sulfur, which feed is passed at reforming conditions through the series of metal promoted, platinum-catalyst containing reactors. This maximizes catalyst activity, catalyst stability, and C.sub.5.sup.+ liquid yields. (It may also protect the unit in the event of a hydrofiner upset which may feed additional sulfur into the unit.) On the other hand, however, it is well known that exotherms occur when an unsulfided catalyst is swung on oil without sulfur in the feed or recycle gas. Accordingly, at the time a reactor containing the unsulfided catalyst is swung on oil, a sufficient amount of unhydrofined sulfur-containing feed is fed directly into the unit to sulfide the unsulfided catalyst of the reactor placed on oil. An amount of the un-hydrofined sulfur-containing feed sufficient to introduce from about 0.6 to about 10 ppm, preferably from about 0.6 to about 5 ppm, sulfur into the feed is adequate. Immediately after the unsulfided catalyst is sufficiently sulfided by the by passing unhydrofined feed, the flow of feed by passing the hydrofiner is interrupted and the unit is again operated at essentially zero sulfur conditions. This method of operation is particularly advantageous in the operation of cyclic (or semi-cyclic) units which do not contain driers. Sulfur is contained in the reforming unit only during catalyst swings, and its concentration in the unit is negligible at all other times. In effect, the average sulfur level over the length of the cycle is minimized. The advantages of this over presulfiding catalyst in the regeneration circuit is that large bursts of sulfur do not occur to the same extent such as would cause heavy loads of hydrogen sulfide for removal by the driers. This process thus allows optimum sulfur control of the unit at all times. Hence, one maximizes yield and catalyst activity performance for each unit on an individual basis. The implications of these improvements are particularly important for units which operate without driers, and it is well known in the art that driers are not reliable for sulfur control. Hence, this process becomes particularly important as drier performance deteriorates. It is thus known that without driers a reactor swing is required about each 71 hours, whereas this process maintains the unit near zero sulfur except for about 0.1 to about 2 hours prior to and during reactor swings; and then only a small amount of sulfur need be purged from the unit after a swing.
These features and others will be better understood by reference to the following more detailed description of the invention, and to the drawing to which reference is made.